1. Field of the Invention
The present invention relates to communication systems. In particular, the present invention relates to the testing of communications system components such as cable modems.
2. Description of Related Art
As information becomes increasingly more available on communication networks such as a LAN or over the Internet, the development of new methods and apparatus for sending and receiving this information more quickly between communication system users has become an important issue. For instance, one-way and two-way cable modems, both internal and external, based on the Multimedia Cable Network System (MCNS) Data-Over-Cable Interface Specifications (DOCSIS) standard, are currently available to consumers to access data over the Internet at speeds far in excess of those previously attainable by standard analog telephone modems. An external cable modem is a complete, self contained unit which is housed in its own enclosure, separate from a personal computer (PC), as opposed to an internal cable modem which is designed as a peripheral card on a printed circuit board (PCB) inserted into a PC. Two-way cable modems receive modulated data from a head-end (H/E) controller over a 75-ohm coaxial cable (the same cable found in residential housing) and send back upstream data over this same cable to the H/E controller. A one-way cable modem receives data from the H/E on a 75-ohm cable, but transmits upstream data back to the headend using a standard analog telephone modem (i.e. 28/33/56 kbps). In each case the H/E controller exists to serve a number of subscribers to the cable modem service.
Downstream (D/S) data for all subscribers is interleaved in time and continuously transmitted down the cable. The downstream data in one instance occupies a 6 MHz wide channel with a center frequency between 54-850 MHz. Raw D/S data rates may range between 30-40 Mbps. However, most subscribers will see much less than this since the downstream bandwidth needs to be shared with many other subscribers as stated earlier. A typical cable plant installation will have between 500 and 2000 subscribers on a particular downstream channel. In addition, there is some degree of overhead required for header data and forward error correction. This serves to lower the true raw data rate somewhat.
In the case of D/S data, each cable modem continuously monitors the D/S channel. When data addressed to a particular modem is received, the modem takes appropriate action. All other data which is not addressed to that modem is ignored. In the case of the two-way cable modem system, all replies are transmitted on the upstream (U/S) channel of the coaxial cable back to the H/E controller. In one instance of the typical two-way cable modem system, there is no contention (or collisions) on the D/S channel, because no modem ever uses the D/S data channel frequency for U/S data. For, in this instance of the system, the U/S data occupies channels from 200 kHz-3.2 MHz wide in the range of 5-42 MHz. The H/E controller is the single system component which completely decides what data to what modem is sent when on the D/S channel.
However, in the case of the U/S data channel for a two-way system with a number of subscribers there are many cable modems which must compete with each other in some fashion to send their data back to the H/E controller. Of course, if two modems try and send data at the same time to the H/E controller, a collision can occur. Unlike a typical network such as an Ethernet, the individual cable modems can not xe2x80x9chearxe2x80x9d (i.e. receive or monitor) data from other cable modems. This is due mostly to the one-way transmission property of the cable plant (due to directive circuit elements, such as power splitters, amplifiers and directional couplers) and also due to the large time delays inherent in the cable plant due to the large distances involved in the cable routing. FIG. 1 shows a diagram of a typical cable plant. The typical cable plant includes a headend controller 100 which is coupled to the rest of the plant via, in one instance, fiber optic cable 110. Data is passed from the headend 100 to the cable modems such as modems 1, 2, 3, 4, N, and N+1, via a network of combiners such as 2-way combiners 115, and 4-way combiners 120. Similarly, in a two-way system, data is passed from the cable modems to the headend 100 over the same network. Additionally, in some existing cable modem plants, the U/S data is split-off from the cable at the fiber 110 junction.
Therefore, it is up to the H/E controller to decide which subscriber modem sends U/S data at what time. In one instance this is done by using a system of mini-slot time increments of around 6.25 use each. Each modem is assigned a time in which it can transmit its signal so as to arrive at the H/E controller in time-interleaved fashion, thereby not colliding with U/S data from other modem subscribers. For all of this to work, the H/E controller performs a ranging operation to determine the time delay from each modem. The H/E controller then figures out for each modem a time slot in which it can send its data so as to not collide with the U/S data from other modems at the H/E controller. The details of this process are complicated and are described more fully in the MCNS DOCSIS specifications referred to earlier.
As can be seen from the above discussion, in order for the overall cable modem based communication system to work properly, especially the two-way cable modem system, each cable modem in the system must be operating properly and according to the MCNS DOCSIS specifications. Thus, in order to ensure a robust cable modem based communication system, it is imperative that each individual cable modem in the system be properly tested to ensure that it is operating correctly.
Currently known methods of testing cable modems, either on the factory floor before home installation, or in the field at the end users installation site, generally make use of bulky, expensive and complicated headend test equipment as illustrated in FIG. 2. A headend unit 205 is coupled to a cable modem 200 which is in turn coupled to a computer 210. The headend 205 is a complex computer controlled apparatus which can be placed in a test mode to send and receive data from a cable modem 200 being tested to analyze the cable modem""s performance. Thus, in order to test a number of modems each modem would be brought to and connected to a headend where a series of tests would be runxe2x80x94the modem would then be disconnected and the next modem would go through the same process. Derivations of this headend testing methodology could involve connecting ten or more modems 200 up to the same headend 205 which could sequentially run the testing process on each of the modems.
There are a number of disadvantages with these testing approaches. First, the use of a headend test unit either in the field or on the manufacturing floor is very expensive because of the high cost of the relatively complicated headend test unit. Further, because of the complication of the headend test units, highly skilled test technicians are needed to operate them, even if only a relatively simple test needs to be performed. Finally, the use of a headend test unit takes a large amount of time to setup and perform the test, especially when only a simple functionality test of the unit under test is required. For instance, it is desirable to be able to perform a series of power on self tests which test a number of simple operations of the modem prior to performing any further detailed tests.
Therefore, what is needed is a new method and apparatus which is capable of performing operability tests on a cable modem in a quick, efficient, and cost effective manner which avoids the aforementioned problems of currently known testing methods.
As discussed above, currently known methods of testing the operability of the RF hardware components of a cable modem can be expensive and complicated, especially when only a simple power on self test is desired. This is because the headend devices used in currently known methods are large and cumbersome devices that are designed for purposes beyond simple testing. Similarly, these devices require a skilled technician to operate. With the above in mind, a method and apparatus for testing a cable modem which is simpler, faster, and more economically efficient than currently known methods is needed.
Accordingly, in one embodiment of the present invention a method of performing an operability test on a communications system device is provided. In this embodiment, a set of output test data is provided, and an output signal is generated in response to this set of output test data with the communications system device. This output signal is provided to a reflective mixer as an input signal, and the reflective mixer generates a reflected signal in response to the output signal. Finally, this reflected signal is provided as a second input signal to the communications system device.
In a further embodiment, the method may also include the steps of generating at least one set of input test data in response to the second input signal and comparing at least one set of input test data with the set of output test data. In this manner the operability of the communications system device can be tested. A further embodiment of this method may include the step of providing an indication of whether the communications system device either passed or failed the operability test.
In one embodiment of the above method steps, the steps of comparing the set of input test data with the set of output test data and providing an indication of whether the communications system device either passed or failed the operability test, are performed by a machine executing a program of instructions tangibly embodied in a program storage device readable by the machine.
In another embodiment of the above method steps the communications system device comprises a cable modem comprising a modulator, a tuner, and a demodulator. In a further characterization of this embodiment, the output signal comprises a modulated signal generated by the modulator, such as a QPSK and a 16-QAM modulated signal. In a still further characterization the output signal comprises a modulated signal at a center frequency from 5 to 42 Megahertz. In a further embodiment, the demodulator generates the at least one set of input test data. Still further, the method may include providing an indication of whether the modulator, the tuner, and/or the demodulator either passed or failed the operability test.
In still another embodiment of the invention, the reflected signal comprises a set of information signals each of which comprise a signal that is essentially the same as the output signal. Further, each information signal is centered about a frequency that is essentially equal to a harmonic of a sample frequency plus or minus the center frequency of the output signal. In one embodiment, the center frequency is from 5-42 Megahertz, and the sample frequency is 100 Megahertz.
This embodiment may be further characterized by the additional method steps of generating a plurality of sets of input test data in response to the second input signal, wherein each of the sets of input test data corresponds to a particular information signal and comparing at least one set of input test data with the set of output test data. Still further, this embodiment of the method may also include the steps of comparing at least one set of input test data to at least one other set of input test data, and providing an indication of whether the communications system device either passed or failed the operability test.
Still further, this embodiment may be defined in that the steps of comparing the plurality of sets of input test data with the set of output test data, comparing at least one set of input test data to at least one other set of input test data, and providing an indication of whether the communications system device either passed or failed the operability test may be performed by a machine executing a program of instructions tangibly embodied in a program storage device readable by the machine. The step of generating a plurality of sets of input test data in response to the second input signal may be performed by the demodulator in response to command signals generated by the machine executing the program of instructions. Similarly, the step of providing a set of output test data may also be performed by the modulator in response to command signals generated by the machine executing the program of instructions.
In still another embodiment of the method, the method may include the steps of coupling a spectrum analyzer to the reflective mixer; and analyzing the output signal with the spectrum analyzer.
The present invention, as summarized above with respect to method steps, may be alternatively characterized as a communications system testing apparatus. The testing apparatus includes, in one embodiment, a communications system device comprising a transmitter and a receiver, an output node electrically coupled to the transmitter, wherein the output node receives an output signal generated by the transmitter in response to a set of output test data, and a reflective mixer electrically coupled to the output node, wherein the reflective mixer generates a reflected signal in response to the output signal. Also provided is an input node electrically coupled to the reflective mixer and to the receiver, wherein the input node receives the reflected signal, and a test control device, electrically coupled to the communications system device and configured to control a test mode of the communications system device such that in the test mode the transmitter generates the set of output test data in response to command signals generated by the test control device.
In a further characterization of this embodiment, the receiver generates at least one set of input test data in response to the reflected signal, which in a further embodiment is generated in response to command signals generated by the test control device. In a still further embodiment, the test control device performs a test of the operability of the communications system device by comparing the at least one set of input test data with the set of output test data. In one instance, the test control device generates an indication of whether the communications system device either passed or failed the operability test.
In one embodiment of the invention the communications system device comprises a cable modem. In this instance, the transmitter comprises a modulator and the receiver comprises a demodulator. The output signal, in this instance, may comprise a QPSK or a 16-QAM modulated signal with a center frequency from 5 to 42 Megahertz.
In one characterization of the above embodiment, the receiver generates the at least one set of input test data in response to command signals generated by the test control device, and the test control device performs a test of the operability of the communications system device by comparing at least one set of input test data with the set of output test data.
In another embodiment of the apparatus the reflected signal comprises a set of information signals each of which comprise a signal that is essentially the same as the output signal. Each information signal is centered about a frequency that is essentially equal to a harmonic of a sample frequency plus or minus the center frequency of the output signal. In one instance, the center frequency is from 5-42 Megahertz, and the sample frequency is 100 Megahertz.
In one instance of this embodiment, the receiver generates a plurality of sets of input test data in response to the reflected signal, wherein each of the sets of input test data corresponds to a particular information signal. In one case, the receiver generates the plurality of sets of input test data in response to command signals generated by the test control device. In another case, the test control device performs a test of the operability of the communications system device by comparing at least one set of the plurality of sets of input test data with the set of output test data. In still another case, the test control device performs a test of the operability of the communications system device by comparing at least one set of the plurality of sets of input test data with at least one other set of input test data.
In another embodiment, the apparatus also includes a spectrum analyzer electrically coupled to the reflective mixer such that the output signal can be analyzed by the spectrum analyzer. In still another embodiment, the test control circuit comprises a machine executing a program of instructions tangibly embodied in a program storage device readable by the machine.
Lastly, in another embodiment of the invention, the reflective mixer includes a signal generator which generates a switching signal at a sample frequency and a variable impedance device configured to adjust one of the phase and amplitude of the reflected signal in response to the switching signal. In alternate embodiments, the switching signal may comprise one of a train of pulses and an overdriven sine wave at a sample frequency of 100 Megahertz. In one embodiment the variable impedance device comprises an FET transistor wherein the switching signal is provided to the gate of the FET transistor.